Semiconductor components having a high dielectric strength, i.e. semiconductor components having dielectric strengths of from a few tens of volts (V) to a few kilovolts (kV), are widely used in many fields, such as, for example, industrial electronics, automotive electronics or consumer electronics. Semiconductor components having a high dielectric strength which are able to carry high currents, such as, for example, currents of a few amperes or more, in the on state are also designated as power components. Semiconductor components having a high dielectric strength include, for example, MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), IGBTs (Insulated Gate Bipolar Transistors), bipolar transistors, bipolar diodes, thyristors or Schottky diodes.
These components have a relatively lightly doped semiconductor region, usually designated as the drift region (in the case of MOSFETs) or as the base region (in the case of diodes or thyristors). This drift region/base region forms a pn junction or a Schottky junction with another component zone, such as, for example, a body region in the case of a MOSFET or an IGBT, and is able to take up a space charge zone in the case of a reverse-biased pn junction/Schottky junction. The reverse voltage strength, that is to say the voltage which can maximally be applied in the reverse direction before a critical field strength is attained and an avalanche breakdown commences, is dependent, inter alia, on a doping concentration of the drift region/base region and the dimension thereof in a direction perpendicular to the pn junction/Schottky junction.
In a semiconductor component having a high dielectric strength, the drift region/base region occupies a significant part of the volume of a semiconductor body in which the semiconductor component is implemented. This applies in particular to a vertical semiconductor component, that is to say a component in which the drift region/base region is arranged between further component zones (for example the body zone and the drain zone in the case of a MOSFET) situated in the region of opposite sides of the semiconductor body. For the production of such a semiconductor component, therefore, it is desirable to have available a semiconductor substrate having a basic doping that already corresponds to the desired doping of the drift region/base region. Further doped component regions can then be produced by conventional doping methods in the semiconductor substrate, those regions in which the basic doping is maintained forming the drift region/base region.
On account of the abovementioned dependence of the dielectric strength of the component on the doping of the base region/drift region, providing a semiconductor substrate having an exactly defined basic doping is of great importance.
In order to reduce costs when producing semiconductor components, usually a multiplicity of identical components are produced simultaneously on the basis of a semiconductor wafer. Said semiconductor wafer forms a semiconductor substrate for a multiplicity of components and is divided into individual semiconductor chips (referred to as dies) after processing.
Such semiconductor wafers for the production of semiconductor components are obtained by sawing a cylindrical (rod-shaped) single crystal. Known methods for producing such a single crystal include the Czochralski (CZ) method, the magnetic Czochralski (MCZ) method or the float-zone (FZ) method. The single crystal can be doped during the production method. In this case, by means of the FZ method, a single crystal having a very homogeneous and defined doping can be produced, which can be subdivided into semiconductor wafers suitable as substrates for the production of high-voltage components. However, heretofore, single crystals produced according to the FZ method have been available only with a diameter of 8″ (inches). In order to increase efficiency, however, it would be desirable to process semiconductor wafers having a higher diameter, such as 12″, for example, in order to be able to produce a higher number of components simultaneously.
Heretofore, however, single crystals having such higher diameters have not been able to be produced according to the FZ method. Although such single crystals can be produced by the MCZ method, the single crystal is already doped during the production method, thus resulting in a very inhomogeneous doping which decreases greatly from a first longitudinal end to a second longitudinal end of the semiconductor rod. Furthermore, the maximum doping present at the first longitudinal end can also fluctuate from single crystal to single crystal under identical production conditions.